In fatwa no 406201, it is stated the impermissiblity to conduct transactions on someone else's property without permission, it is only prohibited if i open a
Stand there, or its prohibited in all case?.
Example is like:
1. i ever do online transaction on an online
Shop when im at someone else's house, and i didnt ask for permission to conduct transaction.
2. I used to do cash on deliveries, so me and the one who wants to buy my stuff (or vise versa) gather on a public or private place, then the potential buyer examine the item he/she wants to buy there (while maybe the seller explain some stuff about the item). Then the potential buyer choose to continue or cancel the transaction. And used not to ask permission when doing this.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It appears that there is nothing wrong in what have mentioned, and it falls under the category of customary permission. There are two categories of permission: explicitly expressed permission and customary (implicit) permission. The explicitly expressed permission is when an owner of a property says to another person, “I give you my permission to do such-and -such (with my property),” while the customary permission is when it is known, on the basis of the implicit customary indication, that the owner would not object to such action or disposal of his property, and he has not communicated any verbal prohibition of it to the other party. This rule was mentioned by Shaykh Al-Islaam Ibn Taymiyyah in his book Al-Qaawaa‘id An-Nooraaniyyah and by his student Ibn Al-Qayyim, may Allaah have mercy upon him, in his book I‘laam Al-Muwaqqi‘een.

In one of his statements, Ibn Taymiyyah said: “The customary permission to use, transfer ownership, or dispose of a property through Wakaalah (i.e. appointing someone to act on someone else's behalf, whether the appointed is an agency or an individual) is as good as the verbal permission. Both Wakaalah and permission of use take effect by an indicative statement or action. Moreover, knowledge of the owner’s approval of a given action or disposal is held in the same standing as showing his explicit consent to it…”

Selling commodities in a store is not the same as doing so in a house. It is customarily known that only the commodities of the store owner are sold therein, and therefore, none else has the right to come to his store and sell any commodity in it without the owner’s permission. As for houses and public places, this may occasionally take place there without rendering them designated places for trade, and it may not require verbal permission; a customary permission is adequate in this regard.

Allah Knows best.